SUID helper executable

The SUID helper binary is called chrome_sandbox and you must build it separately from the main ‘chrome’ target. Chrome now just assumes it's next to the executable in the same directory. You can also control its path by CHROME_DEVEL_SANDBOX environment variable.

In order for the sandbox to be used, the following conditions must be met:

The sandbox binary must be executable by the Chromium process.

It must be SUID and executable by other.

If these conditions are met then the sandbox binary is used to launch the zygote process. Once the zygote has started, it asks a helper process to chroot it to a temp directory.

CLONE_NEWPID method

The sandbox does three things to restrict the authority of a sandboxed process. The SUID helper is responsible for the first two:

The SUID helper chroots the process. This takes away access to the filesystem namespace.

The SUID helper puts the process in a PID namespace using the CLONE_NEWPID option to clone(). This stops the sandboxed process from being able to ptrace() or kill() unsandboxed processes.

In addition:

The Linux Zygote startup code sets the process to be undumpable using prctl(). This stops sandboxed processes from being able to ptrace() each other. More specifically, it stops the sandboxed process from being ptrace()'d by any other process. This can be switched off with the --allow-sandbox-debugging option.

Limitations:

Not all kernel versions support CLONE_NEWPID. If the SUID helper is run on a kernel that does not support CLONE_NEWPID, it will ignore the problem without a warning, but the protection offered by the sandbox will be substantially reduced. See LinuxPidNamespaceSupport for how to test whether your system supports PID namespaces.

This does not restrict network access.

This does not prevent processes within a given sandbox from sending each other signals or killing each other.

Setting a process to be undumpable is not irreversible. A sandboxed process can make itself dumpable again, opening itself up to being taken over by another process (either unsandboxed or within the same sandbox).

Breakpad (the crash reporting tool) makes use of this. If a process crashes, Breakpad makes it dumpable in order to use ptrace() to halt threads and capture the process's state at the time of the crash. This opens a small window of vulnerability.

setuid() method

This is an alternative to the CLONE_NEWPID method; it is not currently implemented in the Chromium codebase.

Instead of using CLONE_NEWPID, the SUID helper can use setuid() to put the process into a currently-unused UID, which is allocated out of a range of UIDs. In order to ensure that the UID has not been allocated for another sandbox, the SUID helper uses getrlimit() to set RLIMIT_NPROC temporarily to a soft limit of 1. (Note that the docs specify that setuid() returns EAGAIN if RLIMIT_NPROC is exceeded.) We can reset RLIMIT_NPROC afterwards in order to allow the sandboxed process to fork child processes.

As before, the SUID helper chroots the process.

As before, LinuxZygote can set itself to be undumpable to stop processes in the sandbox from being able to ptrace() each other.

Limitations:

It is not possible for an unsandboxed process to ptrace() a sandboxed process because they run under different UIDs. This makes debugging harder. There is no equivalent of the --allow-sandbox-debugging other than turning the sandbox off with --no-sandbox.

The SUID helper can check that a UID is unused before it uses it (hence this is safe if the SUID helper is installed into multiple chroots), but it cannot prevent other root processes from putting processes into this UID after the sandbox has been started. This means we should make the UID range configurable, or distributions should reserve a UID range.

Future work

Having the SUID helper as a separate project should make it easier for distributions to review and package.

Possible extensions

History

Older versions of the sandbox helper process will only run /opt/google/chrome/chrome. This string is hard coded (sandbox/linux/suid/sandbox.cc). If your package is going to place the Chromium binary somewhere else you need to modify this string.